Automatic volume control system



Oct. 29, 19.35. K. A. cHlTTlcK AUTOMATIC VOLUME CONTROL SYSTEM Filed Aug. 20, 1952 at Nm.

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SNN E Patented Oct. 29, 1935 2,019,173 AUTOMATIC VOLUME CONTROL SYSTEM Kenneth A. Chittick, Haddon Heights, N. J., as-

signor to Radio Corporation of America, New

York, N. Y.,

a corporation of Delaware Application August 20, 1932, Serial No. 629,651

6 Claims.

The present invention relates to automatic volume control systems of the type having a thermionic vacuum tube or amplifier arranged to respond to variations in the strength of a modulated carrier wave or to the amplitude of a signal wave to control the amplification in a signal amplifying channel or to control the amplitude of the signals transmitted through a signal transmission substantially constant independently of variations in the strength of the modulated carrier wave or of the amplitude of signal waves applied thereto.

The invention relates more particularly to automatic volume control systems for radio receiving apparatus and the like wherein a plurality of thermionic or electric discharge devices, such as vacuum tubes, are employed for amplifying, detecting, and controlling received modulated signal Waves and wherein a common source of operating potentials is provided.

In apparatus of the character referred to, the source of operating potentials is ordinarily a potential divider resistor supplied with unidirectional current through a suitable rectifying means from an alternating current source, with filtering means interposed between the rectier means and the potential divider resistor.

The filter means ordinarily includes a filter choke coil which may be the field winding of the output or sound producing device employed with the receiving apparatus, such for example as the field winding of a dynamic loud speaker. In any event, the filter means may have an appreciable resistance, sufcient to produce an appreciable potential drop.

Heretofore this potential drop has been utilized in some instances for the purpose of supplying grid bias potentials to certain of the vacuum tubes in the apparatus, such as the output tubes for example, but in such cases only a portion of the potential drop is ordinarily utilized, for the reason that the potential available is ordinarily greater than that required for bias purposes.

With the advent of improvements in radio receiving apparatus and the like, automatic volume control devices have come into extensive use and at present it is common practice to provide a vacuum tube in the receiving system for automatic volume control purposes. This tube is ordinarily connected into thesystem in such a manner that a variation in signal potential applied to it causes a variation in its plate current which is translated into a controlling potential for the grids of certain (Cl. Z50- 20) of the amplifying tubes through a plate resistor with which the grid circuits of the controlled tubes are connected.

In the interconnection of the controlled and the controlling tube it has been found that additional potential must be provided for the automatic volume control tube and this has caused considerable difficulty in the design of radio receiving apparatus to provide an effective automatic volume control system Without increasing 10 the cost of the apparatus appreciably, because of the additional potentials required for the automatic volume control tube and the means for supplying such potentials.

A further problem has been presented to the radio art by the advent of certain new types of amplifying tubes which, while having improved operating characteristics, such as a high ampliflcation constant and reduction in distortion, also require, in an automatic volume control system, a relatively wide range of controlling potentials for effective control of the amplification of the signals by means of a controlling or volume control vacuum tube.

The well known and usual means for controlling the amplification or sensitivity of a radio receivingsystem embodying an automatic Volume control tube is constituted by a grid bias control device for the automatic volume control tube. With modern amplifying systems embodying amplifier tubes having a high amplification constant and requiring a relatively wide range of grid control potential, this arrangement for controlling the sensitivity of a receiving system and the operation of the volume control tube has been found to be impractical in some instances and wholly inadequate in others.

Accordingly, it is an object of the present invention to provide an improved automatic volume control system that shall have a relatively wide range of control suiiicient to provide a wide range of controlling potentials for application to any signal channel or signal amplifying channel wherein such wide ranges of controlling potentials are required effectively to control the ampli- '45 iication or signal transmission therethrough.

It is a further object of the present invention to provide an automatic volume control system of the above referred to character that shall be adapted to provide a wide range of controlling potentials when energized from a source of potentials of limited value.

It is a still further object of the present invention to provide an automatic volume control system embodying a thermionic vacuum tube having a control grid, an anode, and a cathode, and means therein for simultaneously controlling both the anode and the grid potentials in aiding relation to each other, whereby a controlling potential in the anode circuit may be caused to vary over a relatively wide range in connection with a potential supply Source providing a limited available potential. l

It is well known'that for purposes of economical design and operation, the potential drop available in the .filtering means in a radio receiving apparatus for example, is caused to be relatively low and the potential available therefrom is, therefore, limited in value. It has been found, however, that in accordance with my invention this limited available potential may be utilized effectively and fully to provide a relatively wide range of control for an automatic volume control tube.

In carrying out my invention I connect the grid and anode circuits of an automatic volume control tube to the terminals of the limited source of potentials, such as the filter means in a radio receiving supply circuit, .and connect across, in shunt, between said terminals, a potentiometer resistor, the movable contact of which is connected with the cathode of said automatic volume control tube. By this arrangement the cathode is caused to iioat between the grid terminal of the potentiometer resistor and the anode terminal thereof. It will be seen that by this arrangement, the grid and anode potentials may be varied simultaneously between a position of zero grid potential and maximum plate current and a position of maximum grid potential and zero plate current.

It will further be seen that by this arrangement, the simultaneous control of grid and anode potentials is effected through` a single instrumentality and that in the adjustment of such instrumentality, for example the simple potentiometer device above mentioned, simultaneous adjustment of the grid potential and of the plate potential or current is additive or accumulative, whereby a rapid as well as wide range of control is effected, in the plate current available for control purposes in the anode circuit of the automatic volume control tube for example, such as across an anode resistor.

The invention will, however, be better understood from the following description when taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing, Figure 1 is a schematic wiring diagram of a thermionic vacuum tube amplifier and potential supply means provided with an automatic volume control system embodying the invention;

Fig. 2 is a similar wiring diagram of a portion of a radio receiving system embodying a thermionic vacuum tube amplier and an automatic volume control system embodying the invention substantially as employed in a radio receiver; and

Fig. 3 is a schematic wiring diagram of an automatic volume control system representing a modification of that shown in Fig. 2 and embodying the invention. In the above described figures of the drawing, it should be understood that Fig.

l represents the basic automatic volume control circuit embodying my invention in its most simpliiied form and without the interposition of circuit elements for signal transmission purposes, while Figs. 2 and 3 show complete practical embodiments thereof in signal transmission circuits.

Referring to Fig. l, 5 is an automatic volume control tube having a control grid 6, a cathode 1 and an anode 8. The tube may be of any suitable type having the above elements or their equivalents and is provided with a suitable input 5 or grid circuit represented by the grid resistor 9 across which are connected signal input terminals IB through a coupling capacitor I I.

The anode or output circuit of the automatic volume control tube includes a series anode or 10 coupling resistor I2 to the anode or high potential end of which may be connected the circuit to be controlled, which, in the usual radio receiving apparatus, is a grid circuit or a plurality of such circuits for amplifying tubes. 15

In the present example, the high potential end of the coupling or anode resistor I2 is connected vwith the control grid I3 of the signal amplifying tubev I4 through the medium of a conductor I5 in which, it will be understood, in practice may 20 be connected signal transmission means omitted herein for the purpose of simplifying the drawing and for aiding in a clear understanding of the invention.

The amplifying tube I4 and other tubes of the 25 apparatus (not shown) are supplied with operating potentials from a suitable source such as terminals I6, which in the present example are supplied with unidirectional current as indicated by the polarity marks opposite said terminals. 30"

Operating current supplied to terminals i6 at a suitable potential is caused to provide operating potentials in a supply circuit including a potential divider resistor I'I with which said terminals are in circuit through suitable filter means, in- 35 cluding a filter choke or impedance I8 and suitable by-pass filter condensers I9, forming the well known ladder filter. The amplifying tube or tubes receive their operating potentials from the potential or voltage divider resistor from suit- 40 able taps as indicated by the leads 29, extending thereto from the amplifier tube I4.

The amplifying tube I4 may be of any suitable type adapted to be controlled by an automatic volume control system and, in the present ex- 45v ample, is represented by a screen grid tube which may be of the high-mu type providing a relatively high range of amplification and requiring on its control grid a relatively wide range of controlling potentials in order to control its amplification be- 50 tween a high maximum value and substantially zero as is required for automatic volume control purposes. For this reason a relatively wide range of controlling potentials is required in the output or anode circuit of the automatic volume control 55 tube 5 and in the present example, across the output or coupling resistor I2.

To provide such a wide range of controlling potentials which, in connection with modern radio amplifying tubes, may extend from approximately 3 volts negative to substantially 8O volts negative as the bias control range, for example on the tube I4, I connect between the input or grid circuit of the automatic volume control tube and its output circuit, a potentiometer resistor 65 2| and provide a connection between the movable or sliding contact thereof 22 and the cathode 'I of the automatic volume control tube, substantially as shown in the drawing. I then provide means for applying to the terminals 23 and 2li of the potentiometer resistor 2 I, a low potential having a polarity as indicated, whereby a positive potential is applied to the anode circuit and a negative potential is applied to the grid circuit when [5 the sliding or movable contact 22 is located at a point between the terminals 23 and 2.4.

I have found that the filter impedance provided in a radio receiving apparatus of the usual construction is, by the above described arrangement, suflicient to fully operate the automatic volume control tube and when located in the negative lead of the potential supply circuit, such as the impedance or choke coil I8 of the present example, that the potential available across said impedance may be utilized for automatic volume control purposes fully and Without loss of potential or other operating diii'culties in the system.

Accordingly, I prefer to connect a major portion of the potential drop producing means of a lter system for the power' supply circuit of the apparatus, in the negative lead of such circuit and to utilize such potential drop producing means as a source of operating potentials for an automatic volume control tube in my improved system.

'I'his connection may readily be made, as shown in the present example, but circuit leads 25 which provide a parallel connection between the potentiometer resistor 2l and the potential drop producing lter or impedance means I8. The potential drop available across the impedance means I8`is, in practical embodiments of the invention, in the usual radio receiving apparatus, substantially between 80 and 100 volts and by my improved control arrangement may be applied to the automatic volume control tube all as grid bias potential or all as anode potential Without grid bias, and as part grid bias and part anode potential at intermediate values, thereby providing a maximum range of control of output potentials available across the output resistor l2 of the automatic volume control tube 5.

It will be seen that an automatic volume control system of the type above described involves a minimum of circuits, and, because of its simplicity is readily adapted for connection in existing radio receiving apparatus and the like without involving changes in wiring connections already existing for supply purposes, since the automatic volume control tube is connected across a potential source not ordinarily fully utilized and not in the main potential supply circuit. Since the automatic volume control tube is interconnected with the controlled tubes ordinarily by direct connection between its anode circuit and the grid circuit of the controlled tubes, the additional potential drop required for the automatic volume control tube is made available without changes in the design of power supply wherein the matter of providing additional operating potentials for the automatic volume control tube is a serious problem from the design and cost standpoint.

The use of limited available operating potentials for automatic volume control purposes such as are provided by the filter means in a power supply circuit for radio apparatus and the use ofv such limited potentials for the control of wide ranges of grid bias in controlled amplifier tubes of the high-mu type, is made possible by the above described arrangement wherein it will be seen the cathode is caused to float between the grid and the plate or between the grid and the plate connections with the potential supply source.

The simpliiied system above described may have many practical embodiments in radio apparatus embodying signal channels wherein automatic control of the amplitude or of the poten-V tial output is desired, independently of variations in the strength of signals appliedlv to the input thereof, as is the case in radio receiving apparatus particularly.

One practical embodiment of the circuit em- 5 bodying the invention as shown and described in connection with Fig. 1 is shown in Fig. 2, in which 26 is an intermediate frequency amplifying tube of the screen grid equi-potential cathode type, having a high amplification factor, and 21 10; is the automatic volume control tube of the three element equi-potential cathode type. The amplier tube 26 is included in the intermediate frequency amplifier portion of the signal transmission channel of the radio receiver. For this 15. purpose it is provided with a tuned grid or input circuit 28 connected with an intermediate frequency coupling transformer 29 and a tuned output circuit 30 provided with an intermediate frequency output transformer 3! from which the 20, signal channel is extended to the detector (not shown) through leads indicated at 32. The signal channel of the receiving apparatus thus includes the input coupling transformer 29, grid circuit 28, intermediate frequency amplifying 25 tube 25, output transformer 3l, and the output leads 32.

In order to maintain the amplitude of the signals delivered to the detector through output leads 32 substantially constant, independently of 30,V variations in the signal amplitude. applied to the input or grid circuit 28, the automatic volume contro-l tube 27 is connected with the signal channel to receive controlling potentials therefrom, and to this end and by way of example, is con- 355 nected with the secondary of the transformer 3l through a suitable coupling condenser 33 and a grid resistor 3d, forming part of the grid circuit of the automatic volume control tube. The grid circuit terminates in the control grid 35. 40

The alternating current control tube is also provided with an anode 36 anda cathode 3l, the latter being coupled through a suitable coupling resistor 33 with the grid circuit 28 of the intermediate frequency ampliiier 26. A similar connection to additional grids for controlling purposes may be provided in connection with the lead indicated at 39. The resistor 38 is suitably bypassed by a condenser 55. Both the grid and anode circuits are suitably ley-passed to cathode through the medium of by-pass condensers il A radio frequency choke coil .12 serves to prevent the flow of radio frequency currents through the plate circuit and thence to the following amplier tube or tubes.

The amplifier tube 26 receives suitable operating potentials from. a voltage divider resistor 43- lii-rl through anode, screen grid, cathode and control grid connections 4-ll`d3-QSL respectively, all of which are supp-lied with suitable byy pass condensers indicated at 5K1.

The tap connections with the voltage divider resistor are such that the amplifier tube and other tubes of the apparatus may receive the proper operating potentials. It will be noted that the 65. section d5 of the voltage divider resistor is in the grid cathode circuit of the amplifier tube 25 and supplies -a proper negative operating potential thereto. It will also be noted that the anode coupling resistor 3S in the automatic Volume control 70` tube output circuit is returned to the negative side of the voltage divider resistor through the lead 49 which is grounded, as indicated at 5 I. The resistor 38 is, therefore, included in the grid circuit of the amplifier tube or tubes and returns to A ground potential. The' potential drop through the resistor 38 is, therefore, applied to the grids of the amplifier tubes, and the circuit arrangement is such that the potential delivered by the automatic volume control tube 21 tends to increase in response to an increase in signal strength applied to the automatic volume control tube and this potential is an addition to the fixed bias provided by the section 45 of the voltage divider resistor. This action increases the negative bias potential in the grids of the amplifier tubes with respect to cathode, and tends to reduce theamplification provided by the amplifier tube 26, thereby maintaining the volume or the signal amplitude delivered through the output leads 32 substantially constant, as will be seen from a further consideration of the automatic volume control circuit hereinafter described.

The voltage divider resistor 43--44--45 receives current for operation of the various circuits of the receiving apparatus from an alternating current source represented by leads 52 through a suitable rectifier system represented by the power transformer 53 and rectifier tubes 54, whereby unidirectional current is supplied to the voltage resistor, having polarities as indicated.

Interposed between the rectifier and the supply means or voltage divider is a filter system in the negative lead 55 from the rectifier and represented by a fixed resistor 56, a filter choke coil 51 and a second choke coil 53, which last may be provided by any inductive device, such as a loud speaker field. The lter elements above named are provided also with suitable filter condensers 59 which cooperate therewith to filter the current provided by the rectifier 54 as is well known.

The potential drop existing across the filter elements 56--51-58 is utilized to provide operating potenti ls for the automatic volume control tube and to this end the grid circuit of the automatic volume control tube, represented by the grid resistor 34 and the by-pass condenser 4I, is'connected with the negative terminal of the filter system indicated at 60, through a connecting lead t! and a filter means provided by series resistance devices 62 and by-pass condenser 63 to the ground lead 49. Likewise, the anode circuit which includes the coupling resistor 38 and the ground lead 49 is connected to a positive terminal of the filter circuit indicated at 64, while the cathode of the automatic volume control tube is connected with a sliding Contact or variable tap on a potentiometer resistor 66 through a circuit lead 61. The latter lead is preferably suitably by-passed to ground by a by-pass condens` The total available potential in the filter means exists between terminals 60 and 64, which are the negative and positive terminals thereof, respectively. The terminals of the potentiometer resistor indicated at 69 and 1U, respectively, are connected with the terminal 60 and with a tap point 1l between elements 51 and 58 of the filter system, whereby the potentiometer receives only a portion of the total available potential and at the negative end thereof. It will be noted that between the terminal 69,-which is a conjunction point for the lead 6l, and the potentiometer 66 and the supply terminal 6i), filter means including a series impedance device 121 and a filter condenser 13 is interposed, the latter being connected between terminal 69 and the positive lead of the power supply circuit. This represents any suitable filter means for the automatic volume control tube.

In certain instances the plurality of filter means shown in the various supply circuits for the automatic volume control tube, may be omitted, provided that the desired range of control is not required to be sufficiently wide to 5 cause instability or other undesired operating characteristics of the amplifier or automatic volume control tube.

It will be noted that from a comparison of Figs. 1 and 2, in the present embodiment of the auto- 10 matic volume control circuit of the present invention, the potentiometer resistor 66 is so connected that the anode potential on the automatic volume control tube may not be reduced to zero, since the potentiometer is tapped into the con- 15 nection 1l between the terminals 60 and 64 of the filter system. Therefore, the potential available across the iilter choke at 58 represents the minimum anode potential available. However, the grid potential may be reduced to zero. It has 20 been found that in certain systems this is a desirable arrangement tending to reduce instability in operation.

It should be understood that the potential derived from filter elements 56 and 51 jointly, may 25 be derived from a single element as well, the resistor 55. being utilized in the circuit of the present example principally to provide additional potentials other than those provided by the filter element 51. 30

As an example of a system embodying the invention, wherein the entire operating potential for the automatic volume control tube is derived from the field winding of a sound producing device, attention is now directed to Fig. 3 in which 35 the automatic volume control tube is indicated at 14. The input and output circuits 15 and 16, respectively, are similar to those shown in the preceding figure. It will be noted that the automatic volume control tube is of the type known 4,0 commercially as a screen-grid pentode, having a suppressor grid 11 in addition to the control grid 18, screen grid 19, cathode 88 and anode 8l. It has been found that the provision of a thermionic vacuum tube having a cathode and sup- 45 pressor grid, together with a screen grid, results in additional advantages in an automatic volume control circuit embodying the invention, and is at present preferred therein. It has been found that this tube has the advantages that it is more 50 stable in operation and, having a sharp cut off in its grid potential anode current characteristic, provides a wider range of control potentials when employed in the system of the present invention.

In the present example, leads indicated at 82, 55 83 and 84 from the automatic volume control tube are the anode, cathode, and control grid circuits, and are connected, respectively, with the positive terminal 85, variable tap 86, and negative terminal 81 of a potentiometer resistor 88, form- 60 ing the sensitivity of volume control devices in the automatic volume control system substantially as and for the purposes hereinbefore described. The leads 83 and 84 are preferably, although not necessarily, provided with filter means 6o indicated by circuit impedance 89 and by-pass capacitors 96.

It will further be noted that in connection with the potential supply source indicated by leads 9|, in the negative lead thereof, a loud speaker field '(0 winding 92 is connected and serves as a potential source for the potentiometer resistor 88, being connected therewith through a suitable filter comprising a choke coil 93 and a suitable by-pass capacitor 94 to the high potential side of the sup- 75 ply source. Additional filter condensers are indicated at and the potential supply resistor is indicated at 96.

Except for a wider range of control potentials and a more stable operating characteristic, this circuit operates substantially as the circuit described in connection with E ig. 1. The use of a screen-grid, suppressor-grid type of tube permits a variation of the anode potential below that of the screen-grid without setting up conditions of operation ordinarily tending to cause oscillations. It also represents a practical embodiment of the schematic circuit shown in Fig. 1 in connection with an automatic volume control tube of the screen-grid, suppressor-grid type.

From the foregoing description of the various embodiments of the invention it will be seen that my improved volume-control system is readily adapted for use in connection with limited potential supply sources and for the purpose of utilizing substantially all of the existing potential, not only for grid bias control purposes but for anode potential control purposes, and that by means of a suitable impedance device connected between the control grid circuit and the anode circuit, with a floating or variable connection therewith for the cathode, a thermionic vacuum tube automatic volume control device may be caused to provide a relatively wide range of output or controlling potentials, and that such circuit arrangement involves substantially no changes in the power supplying connections in a system having a filter reactor element in the negative potential supply lead.

While the invention has been shown and described in connection with signal amplifying systems for radio receiving apparatus, it should be understood that it may be employed in other apparatus wherein an automatic volume control tube may be utilized for supplying controlling potentials to volume control means therein.

I claim as my invention:

l. In a radio receiving apparatus, the combination with a uni-directional potential supply circuit therefor including a main potential supply network providing relatively high potential drop in said circuit and a series lter impedance device in the negative side thereof providing relatively low potential drop in said circuit, an automatic volume control tube having an anode, a cathode, and a control electrode, means providing a signal input circuit for said tube connected with the control electrode and one terminal of said impedance device, means providing an output circuit for said tube connected with the anode and a second terminal of said impedance device, and means providing a variable tap connection for said cathode between said terminals, said tube having a screen grid and a suppressor grid, whereby the volume control circuit is rendered more stable over a Wide range of operation, and operating potentials for said control electrode, screen grid and anode are provided by said impedance device to the exclusion of the remainder of the anode and grid circuits of said apparatus;

2. In a power supply system, the combination of a potential supply circuit having positive and negative potential supply terminals, a filter choke coil and a voltage divider means connected in series across said terminals, said choke coil providing a low potential drop with respect to the potential drop in said voltage divider means in response to current flow therethrough from said terminals, means for deriving anode, screen grid andv biasing potentials from said voltage divider means, separate means for deriving from said lter choke coil, grid and anode potentials separate and independent of said first named potentials and, in addition thereto, from said supply terminals, and a` control tube having grid and anode electrodes for receiving said last named potentials.

3. In a radio receiving system having a plurality of electric discharge devices including an automatic Volume contro-l device, the combination of meansV for supplying anode and grid operating potentials to said electric dischargedevices exclusive of said automatic volume control device, a lter choke coil in series with said last named means to filter the current flow thereto with relatively low potential drop, and means for utilizing said potential drop across the choke coil to supply anode and grid operating potentials to said automatic Volume control device separately and independently of other electric discharge devices in said system.

4. In a radio receiving system, the combination with a rectiiier having an output circuit, including potential supply means for said system and a filter choke coil in the negative side of said' output circuit adjacent to said rectifier, of an automatic volume control tube having a cathode, an anode, and a control grid, connected with said filter choke coil to receive operating anode and grid potentials therefrom independently `of said potential supply means, and means in said connection for simultaneously varying the grid and anode potentials between a position of zero grid potential and maximum anode current and a position of maximum grid potential and zero anode current, said means comprising a potentiometer resistor connected in parallel with said filter choke coil and having a tap connected with the cathode, a negative terminal thereof being connected. with the grid, and a positive terminal thereof being connected with the anode, said tap being movable along said resistor to and between said terminals to provide a zero grid bias and zero anode supply voltage connection between the cathode and the grid and anode, respectively, and an impedance device in the anode circuit of said volume control tube for supplying controlling potentials to said receiving system.

5. In a radio receiving system, the combination with a potential supply circuit having a filter choke coil providing a limited potential drop in the negative side thereof, and a control circuit for said receiving system requiring a wide range of variable controlling potentials, of means for utilizing said limited source of potential to provide said Wide range of potentials without loss of potential in said supply circuit, said means including an electric discharge device having a cathode, an anode, and a control grid, said anode being connected with a positive terminal of said choke coil, and the control grid being connected with a negative terminal of said choke coil, and means providing a movable electrical tap along said choke coil to and between the terminals thereof for the cathode, whereby the grid bias is reduced to zero when said tap is moved to conv nection with said negative terminal, and whereby connection between said impedance element and said control circuit.

6. In a radio receiving system, the combination of a potential supply circuit therefor having positive and negative potential supply terminals, a lter choke coil and a voltage divider means connected in series across said terminals, said choke coil providing a low potential drop with respect to the potential drop in said voltage divider means in response to vcurrent flow therethrough from said terminals, means for deriving anode, screen grid and biasing potentials from said voltage divider means, automatic volume control means including a control tube having a cathode, grid and'- rst named potentials and, in addition thereto,y

said last named means including a potentiometer resistor connected in shunt relation to said lter choke coil and having a variable tap connection for supplying said last named potentials, and a I'llter connected between said potentiometer resistor and said choke coil including a series impedance element and a by-pass condenser.

KENNETH A. CHITTICK. 

